The present invention relates to a feature-region extraction method and a feature-region extraction circuit and, more particularly, to a feature-region extraction method and a feature-region extraction circuit which are capable of extracting a feature-region of an image for processing the image according to its feature at a pre-processing portion of an image coding device and which are intended to be used, for example, in video telephone systems, video conference systems and so on.
At a motion picture coding and transmitting device for a video telephone, since any transmission line has a limited or finite capacity to transmit a number of bits at a time, it cannot allocate enough bits to an image information. Accordingly, a decoded image may have an impaired picture quality with a mosquito-like distortion, a block distortion and the like. On the other hand, most of the images to be transmitted include an important portion and less important portions. On the basis of this fact, a method is proposed that reduces the subjective deterioration of an image quality by keeping the important portion free from the ill effects of deterioration.
For example, in systems such as video telephone systems, the image of the face region is so important that such systems have been directed toward extracting the face region, preferentially allocating the amount of information transmitted and making the image quality of the face region better, aiming at improvement of the subjective image quality.
The prior art that is concerned with the present invention is disclosed in the publication xe2x80x9cColor motion picture coding device featured by preferentially processing a face imagexe2x80x9d (Hideyuki Ueno, Image Information vol. 24, March 1992, pp. 29-35). The face-region extraction method of the prior art described in the publication will be explained as follows.
The prior art first determines the difference between two neighboring frames of the motion picture inputted to get the interframe differential image and then divides the interframe differential image into rectangular regions. In the respective rectangular regions, the method makes a histogram representing the number of pixels, in which the interframe difference signal is larger than a certain threshold, on an H-axis. The method scans the histogram from the left to right, comparing its values with a certain threshold xe2x80x9cthxe2x80x9d to detect the left and right ends of the moving region.
The image screen is scanned from the upper left end until a first moving region is found. This moving region has a coordinate on a V-axis, which represents a position at the top of one""s head. The image screen is then scanned from the left and the right by a certain range of lower image to determine the left and right ends of the moving region. And then, the outermost positions of the moving region on the H-axis represent the coordinates of the left and right ends of the face. The face length is determined by multiplying the face width by a predetermined coefficient xe2x80x9cxcex1xe2x80x9d. The prior art considers the thus determined image region as a face-region.
As mentioned above, the conventional feature-region extracting method estimates a face-region from an interframe difference signal representing the amount of motion. Consequently, if the image includes any other moving object than the face-region, the prior art method may erroneously take the object as a face part. Furthermore, the method can roughly extract a face-region only in the form of a rectangle, which contour may be emphasized when the regions other than the face are adaptively processed with a low-pass filter. This may impair the subjective image quality.
It is an object of the present invention to provide a feature-region extraction method and the feature-region extraction circuit, which can extract a feature-region exactly along its contour without erroneously extracting any other moving object than the feature one, thereby allowing adaptive processing of portions other than the feature-region without impairing the image quality and can extract a face-region, using color difference components, regardless of the movement and location of a person or a number of persons, improving the subjective image quality.
It is another object of the present invention to provide a feature-region extraction method whereby a small color difference region including a feature color difference component of a feature-region is located within a color difference coordinate system defined by two axes representing two respective color difference signals and an image region, which has a color difference component in the small color difference region mentioned above, is extracted as a feature-region; the small color difference region can take any desired form thereby eliminating the possibility of erroneously extracting any moving object other than the feature-region; the feature-region can be exactly extracted along its contour line to make the image more natural and to have a subjectively improved quality and, therefore, can withstand adaptive processing of other regions with a low-pass filter; and, furthermore, only color difference signals are applied for feature-region extraction to prevent erroneous extraction when the luminance level of the feature-region varies in space and in time.
It is another object of the present invention to provide a feature-region extraction method whereby a small color difference region is a common region of two color difference signals binarized by the use of specified thresholds and a feature-region is easily extracted therefrom.
It is another object of the present invention to provide a feature-region extraction method whereby a feature-region is extracted from a plurality of small color difference regions so as to further improve the image quality.
It is another object of the present invention to provide a feature-region extraction method whereby a feature-region along its contour line is extracted together with a dropout region enclosed within the feature-region contour line, so as to further improve the image quality.
It is another object of the present invention to provide a feature-region extraction method whereby a feature-region is extracted from a common region of two color difference signals binarized by the use of specified thresholds and dropout regions within the feature-region are filled up by using detection patterns weighted by distance so as to further improve the image quality.
It is another object of the present invention to provide a feature-region extraction method whereby in extracting a feature-region by using means for extracting a feature-region and a memory means for storing a feature-region extraction signal, a criterion is given a hysteresis characteristic depending upon whether a pixel backed by L frames (L is a positive integer) and/or its neighbor pixel is a feature-region or not, making it possible to effectively prevent the feature-region from being extracted with flickers and thereby to further improve the subjective image quality.
It is another object of the present invention to provide a feature-region extraction method whereby a feature-region is extracted by using its characteristic, and feature-region extraction signals of plural frames from a memory of M frames (M is a positive integer) are weighted for extracting a weighted feature-region so as to further improve the image quality.
It is another object of the present invention to provide a feature-region extraction circuit whereby a feature-region extraction circuit that includes a means for extracting a main portion of a feature-region and for filling a dropout region can extract a feature-region exactly along its contour line, and thereby adaptive processing of an image region other than the featured one with a low-pass filter can be performed free from the image deterioration.
A feature-region extracting method, according to the present invention, can extract a feature-region by using a small color difference region including a feature color difference component of a feature-region within a color difference coordinate system having two axes representing two respective color difference signals, eliminating the possibility of erroneously extracting any other moving object than the feature-region. Furthermore, the method is capable of exactly extracting a feature-region along its contour, allowing adaptive processing with a low-pass filter to improve the subjective image quality without emphasizing the extracted region""s contour. It binarizes two color difference signals by use of specified thresholds to produce a common region and extracts therefrom a feature-region without erroneously extracting any other moving object than the feature-region.
According to the present invention, a feature-region can be extracted from a plurality of small color difference regions including a feature color difference component of a feature-region, and no other region than the featured one can be extracted even in the case when feature color difference components are widely dispersed within a color difference coordinate system having two axes representing two color difference signals.
In addition, since a feature-region can be correctly extracted along its contour line, it is possible to apply any adaptive processing such as pre-filter processing, which is a low-pass filter processing controlled by an extracted signal, or a quantizing control for roughening the step-size of the quantization of a background region to improve the subjective image quality without unnaturally emphasizing the contour line of the extracted feature-region.
To realize the above-mentioned purposes, the present invention was made to provide:
(1) A feature-region extracting method which determines a small color difference region including a feature color difference component of a feature-region within a color difference coordinate system defined by two axes of two color difference signals and which extracts an image region that has a color difference component within a small color difference region mentioned above as a feature-region;
(2) A means to make a small color difference region be a common region having two color difference signals binarized by certain thresholds and which extracts a feature-region therefrom;
(3) A means to extract a feature-region from a plurality of small color difference regions (in addition to item (1) or (2)); or
(4) A means to extract a feature-region along its contour line together with a dropout region enclosed within the contour line of the feature-region as a feature region; or
(5) A means to binarize two color difference signals of an image by certain thresholds, extract a feature-region from the common region of the binarized color difference signals and detect patterns for blank filling which are suitably weighted according to the distance for filling up the dropout region; or
(6) A means to provide a criterion with an hysteresis characteristic depending upon whether a pixel backed by L frames (L is a positive integer) and/or an adjoining one relating to a feature-region or not and uses it when extracting a feature-region by using a means for extracting the feature-region and a memory means for storing a feature-region extraction signal; or
(7) A means to extract a feature-region by using its characteristic, generate output signals of feature-region extraction of frames from a memory of M frames (M is a positive integer) and make the frame feature-region extraction signals weighted to extract a weighted feature-region; or
(8) A feature-region extraction circuit which comprises a feature-region main-portion extracting circuit for extracting an image region having a feature color difference component of a feature-region and a blank filling circuit for filling up a dropout region which is sandwiched in between the feature-region and which has a different color difference component other than the featured one; and further,
(9) A means to include the feature-region main-portion extracting circuit which comprises a comparison circuit for comparing two color difference signals with the upper limit threshold and the lower limit threshold, and an AND gate circuit for determining the logical product of outputs of the comparison circuit and extracting a common region; and further,
(10) A means to include another feature-region main-portion extracting circuit which comprises a first comparison circuit for comparing two color difference signals with a first upper limit threshold and a first lower limit threshold, a first AND gate circuit for determining a logical product of outputs of the first comparison circuit and extracting a common region, a second comparison circuit for comparing two color difference signals to a second upper limit threshold and a second lower limit threshold, a second AND gate circuit for determining a logical product of outputs of the second comparison circuit and extracting a common region and an OR gate circuit for determining a logical sum of the first comparison circuit""s output and the second comparison circuit""s output and extracting feature regions distributed among a plurality of regions.